Biology subjects

Cruz, S.

Publications and source records attributed to Cruz, S..

10 recordsLinked to original sources

bioRxiv2026

Chloroplast movements in siphonous macroalgae in response to high light and grazing

Fast cytoplasmic streaming enables extensive chloroplast movements in the giant cells of unicellular, siphonous macroalgae. Here, we studied chloroplast movements in two such algae: the Dasycladalean Acetabularia acetabulum and the Bryopsidales Bryopsis sp.. We hypothesised that chloroplast movements function as a protective avoidance mechanism under excess light, particularly in Bryopsis sp., which lacks capacity for fast induction of photoprotective non-photochemical quenching (NPQ) and state transitions. In addition, we also investigated whether chloroplast movements are involved in responses to wounding and herbivory. The movements were studied by light microscopy, photography and pulse modulated chlorophyll a fluorescence quenching analysis. Chemical inhibitors of actin polymerization and microtubules assembly were used to confirm that the observed effects were active responses controlled by the cytoskeleton. A. acetabulum responded to high light by reversible chloroplast aggregation, probed by macro-imaging; and chemical inhibition of chloroplast movements led to an enhancement of Photosystem II photoinhibition, as probed by the fluorescence parameter FV/FM. No chloroplast movements were observed in Bryopsis sp. in response to high light. In A. acetabulum, wounding caused either by cutting or due to feeding by the sap-sucking sea slug Elysia timida triggered aggregation of chloroplasts within minutes of incurring the damage. Interestingly, the aggregation also occurred in intact cells away from the cutting site. Furthermore, the addition of media collected from the vicinity of cut algae was sufficient to induce chloroplast aggregation in intact algae, suggesting that water-borne cues or signals triggered the aggregation response in A. acetabulum. Bryopsis sp., however, responded to cutting by only local chloroplast aggregation. The relevance of chloroplast movements in protection against both abiotic and biotic stressors in A. acetabulum, and the potential reasons behind the different defence strategies of the algae, are discussed.

cell biology↗

bioRxiv2025

Preservation and remodelling of chloroplast lipids in photosynthetic sea slug host cells

The capacity to retain long-term functional algal chloroplasts within animal cells is a singular trait of some Sacoglossa sea slugs. The stolen chloroplasts (kleptoplasts) confer photosynthetic capacity to the host. This study investigated how chloroplast lipidomes from two macroalgae (Acetabularia acetabulum and Bryopsis sp.) are modified after sequestration by the photosynthetic sea slugs Elysia crispata, Elysia viridis, and Elysia timida. Pigments were analysed by HPLC, and lipidomic profiling of chloroplasts (glycolipids, betaine lipids, phytosterols) was conducted using C18-LC-MS/MS. Kleptoplasts retain intact algal pigment profile. Glycolipid signatures in sea slug tissues closely resembled those of their algal donors, although shifts in relative abundances suggest remodelling of plastidial membranes, potentially affecting kleptoplast morphology, particularly membrane curvature and stability. In contrast, betaine lipid composition differed markedly between sea slugs and algal donors, with major betaine species common to all sea slugs. Phytosterol composition was better preserved in sea slugs feeding on Bryopsis sp. than on A. acetabulum. This study demonstrated that kleptoplast lipidomes maintain a strong algal identity but undergo selective modifications within the animal host. This remodelling likely optimizes membrane architecture to support long-term stability and photosynthetic functionality within the hosts cellular environment, facilitating the integration and performance of sea slug kleptoplasts. HighlightThe lipid profiles of photosynthetic sea slugs resemble the algal donors chloroplasts. However, alterations in glycolipid composition suggest that the molecular architecture of kleptoplasts is modulated by the host cells.

biochemistry↗

bioRxiv2025

Provision of phosphates by the host supports kleptoplast functionality in photosynthetic sea slugs

Phosphorus and nitrogen are two fundamental macronutrients needed for photosynthesis, and their balance in photosymbiotic relationships, like those between corals and their zooxanthellae, is known to be highly important for maintaining a healthy partnership between the host and the symbiont. Kleptoplasty is a special case of algae-herbivore interaction, where the host animal or protist sequesters the chloroplasts from their prey algae as kleptoplasts inside their own cells. In some kleptoplastic sea slugs, the kleptoplasts can remain functional for months without the help from the algal nucleus, and the fundamental mechanisms in place to achieve this are still largely unsolved. Unlike in purely symbiotic systems like corals, the importance of nutrient balance to kleptoplast longevity has not been addressed in kleptoplastic sea slugs. We studied the phosphate and nitrogen exchange between the sea slug Elysia timida and its Acetabularia acetabulum derived kleptoplasts in various nutrient depletion and replenishment experiments. Our results show that sea slugs consumed phosphate from their surrounding medium and were able to recover electron transfer rates of kleptoplasts originating from phosphate depleted algae, suggesting that the sea slug provided the acquired organelles with phosphate to the extent that they increased the photosynthetic capacity remarkably well. Additional available phosphate in the water also increased the overall longevity of the kleptoplasts. The sea slugs were not able to recover the photosynthesis of nitrogen-depleted kleptoplasts, and additional dissolved nitrate and ammonium were not beneficial for kleptoplast longevity. Our results suggest that the phosphate-nitrogen balance inside the sea slug cells is mainly limited by phosphate, the addition of which can help the kleptoplasts maintain photosynthesis, perhaps by allowing the use of the glutamine synthetase (GS) and glutamate synthase (GOGAT) nitrogen assimilation pathway to function as an efficient alternative electron sink.

plant biology↗

bioRxiv2025

Siphonous green macroalgae with contrasting capacities for the energy-dependent quenching, qE, rely on different photoprotective mechanisms

To combat light-induced damage, photosynthetic organisms have evolved several photoprotective mechanisms, including non-photochemical quenching (NPQ) of excess light energy. Bryopsidales green macroalgae lack the energy-dependent quenching (qE) and can therefore induce NPQ only slowly. We illuminated a Bryopsidales macroalga (Bryopsis sp.) and a morphologically similar Dasycladales alga (Acetabularia acetabulum), capable of qE, with high light. No differences in the rate of Photosystem II (PSII) damage, probed by the chlorophyll a fluorescence parameter FV/FM in the absence and presence of lincomycin, were observed between constant and fluctuating light in Bryopsis sp., nor between the two algae. In Bryopsis sp., however, photoinhibition lead to decreased rates of electron transfer (estimated by fluorescence and oxygen evolution), while a stimulation was observed in A. acetabulum. Compared to A. acetabulum, Bryopsis sp. showed slow PSII repair, but as the rate of repair increased with increasing rates of the damage, the slow repair may not indicate increased oxidative stress but be a regulatory response. We observed, in A. acetabulum, a post-illumination dip in PSII activity and a concurring bump in NPQ, which were removed by an inhibitor of the NDA2-dependent cyclic electron transfer route and enhanced in the presence of inhibitor of mitochondrial respiration, suggesting that these pathways reduce and oxidise, respectively, the plastoquinone pool in the dark in this alga. Nigericin, which prevents the formation of proton gradient and thus qE, increased photoinhibition in A. acetabulum but not in Bryopsis sp.. Anoxia and inhibitors of the plastid terminal oxidase and mitochondrial respiration, on the other hand, enhanced photoinhibition only in Bryopsis sp., suggesting that oxygen-dependent pathways (including flavodiiron proteins) are important for photoprotection in the qE-deficient Bryopsidales algae.

plant biology↗

bioRxiv2025

Structural flexibility of the human vault protein revealed by high-resolution cryo-EM and molecular dynamics simulations

Vaults are massive ribonucleoprotein complexes, highly conserved and abundant in eukaryotic cells, yet with unclear function. Their thin-walled barrel-shape architecture is composed of two symmetrical, antiparallel half-shells, each containing 39 copies of the major vault protein (MVP). The spacious lumen of the vault suggests a role in cellular transport. To facilitate cargo encapsulation and release, the vault is thought to open into two halves, yet the molecular mechanism governing vault opening remains elusive. Here, we combine cryogenic electron microscopy (cryo-EM) and multi-scale molecular dynamics (MD) simulations to reveal the structural factors giving flexibility to the human vault protein. Using cryo-EM, we identified two alternative conformational states of the human vault, along with the half-vault shell. MD simulations of these structures show extensive, breathing-like motions, porous solvent-exposed surfaces, and distinct structural variability between conformational states. The stable intermediates and the flexibility at the interface of the half vaults together suggest a possible mechanism for the dynamic assembly and disassembly of the vault.

biochemistry↗

bioRxiv2024

Evolution and theft: loss of state transitions in Bryopsidales macroalgae and photosynthetic sea slugs

Green macroalgae within the order Bryopsidales lack the fundamental photoprotective mechanisms of green algae, the xanthophyll cycle and energy-dependent dissipation of excess light. Here, by measuring chlorophyll fluorescence at 77K after specific light treatments, we show that Bryopsidales algae also lack state transitions, another ubiquitous photoprotection mechanism present in other green algae. Certain Sacoglossa sea slugs can feed on Ulvophyceae algae, including some Bryopsidales, and steal chloroplasts -kleptoplasts- that remain functional inside the animal cells for months without the support of the algal nucleus. Our data reveal that the state transition capacity is not retained in the kleptoplasts of the sea slugs, and we provide evidence that the loss is caused by structural changes during their incorporation by the animals. This enforced chloroplast sphericity was observed in all studied kleptoplastic associations, and we propose that it is a fundamental property supporting long-term retention of kleptoplasts in photosynthetic sea slugs.

plant biology↗

bioRxiv2024

Photoprotective mechanisms in Elysia species hosting Acetabularia chloroplasts shed light on host-donor compatibility in photosynthetic sea slugs

Sacoglossa sea slugs have garnered attention due to their ability to retain intracellular functional chloroplasts from algae, while degrading other algal cell components. While protective mechanisms that limit oxidative damage under excessive light are well documented in plants and algae, the photoprotective strategies employed by these photosynthetic sea slugs remain unresolved. Species within the genus Elysia are known to retain chloroplasts from various algal sources, but the extent to which the metabolic processes from the donor algae can be sustained by the sea slugs is unclear. By comparing their responses to high light conditions through kinetic analyses, molecular techniques, and biochemical assays, this study highlights significant differences between two photosynthetic Elysia species with chloroplasts derived from the green alga Acetabularia acetabulum. Notably, Elysia timida displayed remarkable tolerance to high light stress and sophisticated photoprotective mechanisms such as an active xanthophyll cycle, efficient D1 protein recycling, accumulation of heat-shock proteins and -tocopherol. In contrast, Elysia crispata exhibited absence or limitations in these photoprotective strategies. Our findings emphasize the intricate relationship between the host animal and the stolen chloroplasts, highlighting different capacities to protect the photosynthetic organelle from oxidative damage.

plant biology↗

bioRxiv2023

Optogenetic silencing of the Anterior Cingulate Cortex impairs working memory in a delayed non-match to trajectory task

Working memory is a fundamental cognitive ability, allowing us to keep information in memory for the time needed to perform a given task. A complex neural circuit fulfills these functions, among which is the anterior cingulate cortex (CG). Functionally and anatomically connected to the medial prefrontal, retrosplenial, midcingulate and hippocampus, as well as motor cortices, CG has been implicated in retrieving appropriate information when needed to select and control appropriate behavior. The role of cingulate cortex in working memory-guided behaviors remains unclear due to the lack of studies reversibly interfering with its activity during specific epochs of working memory. We used light to activate eNpHR3.0 and thus silence cingulate neurons while animals perform a standard delayed non-match to trajectory task. We found that, while not causing an impairment in eNpHR3.0+ animals when compared to eNpHR3.0-CTRL animals, silencing cingulate neurons during retrieval decreases the mean performance if compared to silencing during encoding. Such retrieval-associated changes are accompanied by longer delays in control animals, consistent with an adaptive recruitment of additional cognitive resources.

neuroscience↗

bioRxiv2022

Different transcriptional responses by the CRISPRa system in distinct types of heterochromatin in Drosophila melanogaster

Transcription factors (TFs) activate gene expression by binding to elements close to promoters or enhancers. Some TFs can bind to heterochromatic regions to initiate gene activation, suggesting that if a TF is able to bind to any type of heterochromatin, it can activate transcription. To investigate this possibility, we used the CRISPRa system based on dCas9-VPR as an artificial TF in Drosophila. dCas9-VPR was targeted to the TAHRE subtelomeric element, an example of constitutive heterochromatin, and to promoters and enhancers of the HOX Ultrabithorax (Ubx) and Sex Combs Reduced (Scr) genes in the context of facultative heterochromatin. dCas9-VPR robustly activated TAHRE transcription, showing that although this element is heterochromatic, dCas9-VPR was sufficient to activate its expression. In the case of HOX gene promoters, although these genes are epigenetically silenced by Polycomb complexes, both were ectopically activated. When the artificial TF was directed to enhancers, we found that the expression pattern was different compared to the effect on the promoters. In the case of the Scr upstream enhancer, dCas9-VPR activated the gene ectopically but with less expressivity; however, ectopic activation also occurred in different cells. In the case of the bxI enhancer located in the third intron of Ubx, the presence of dCas9-VPR is capable of increasing transcription initiation while simultaneously blocking transcription elongation, generating a lack of functional phenotype. Our results show that transcription can be activated in any type of heterochromatin by CRISPRa; nevertheless, its effect on transcription is subject to the intrinsic characteristics of each gene or regulatory element. SignificanceWhether transcription only depends on activating factors binding to chromatin, even though it is found in a silent state as heterochromatin, remains an open question. In this work, we addressed this question using the CRISPRa system via dCas9-VPR as a synthetic transcriptional activator in Drosophila. This activator was directed to a constitutive heterochromatin element and to promoters and enhancers of two HOX genes, which in the tissues where they are not expressed, are present as facultative heterochromatin. In all cases, the CRISPRa system was able to activate transcription, showing that its sole presence is sufficient for this to occur. Although transcription in constitutive heterochromatin was very robust, in the case of promoters and enhancers of HOX genes, the degree of expressivity, penetrance and ectopic effect was different between promoters and enhancers. These results indicate that the presence of a synthetic activator can activate transcription by binding to transcriptional regulatory elements; however, its effect depends on the particular characteristics of each one. These results show how artificial transcription factors can be used to understand transcription regulation at the organismal level.

genetics↗

bioRxiv2021

Photosynthesis from stolen chloroplasts increases sea slug reproductive fitness

Some sea slugs are able to steal functional chloroplasts (kleptoplasts) from their algal food sources, but the role and relevance of photosynthesis to the animal host remain controversial. While some researchers claim that kleptoplasts are slowly digestible snacks, others advocate that they enhance the overall fitness of sea slugs much more profoundly. Our analysis show light-dependent incorporation of 13C and 15N in the albumen gland and gonadal follicles of the sea slug Elysia timida, representing translocation of photosynthates to kleptoplast-free reproductive organs. Long-chain polyunsaturated fatty acids with reported roles in reproduction were produced in the sea slug cells using labelled precursors translocated from the kleptoplasts. Finally, we report reduced fecundity of E. timida by suppressing kleptoplast photosynthesis. The present study provides the first thorough experimental evidence that photosynthesis enhances the reproductive fitness of kleptoplast-bearing sea slugs, confirming the biological relevance of this remarkable association between a metazoan and an algal-derived organelle. TeaserSea slugs incorporate functional chloroplasts from algae and use products of photosynthesis to maximize reproductive output.

ecology↗